The present invention relates to a global integrated communications, navigation and surveillance satellite system.
Current satellite systems provide positioning and time information by broadcasting navigation signals to properly equipped users. For example, a the US Global Positioning System (GPS) consists of 24 satellites orbiting the earth twice a day at an altitude of approximately twelve thousand miles, as well as a network of ground stations to monitor and manage the satellite constellation. The GPS satellites transmit continuous Navigation Data and Ranging (NDR) information 24 hours a day toward the earth. A GPS receiver which properly decodes, tracks and interprets these transmissions from the GPS satellites can compute the position of the GPS receiver as well as determine accurate time. The basic functioning of GPS and GPS receivers, is well known in the art. The GPS satellite system currently broadcasts for civilian use a Standard Positioning Service (SPS) on a single frequency (1575.42 MHz) called L1. The current GPS receivers and the GPS satellites are not capable of two-way communication with each other. GPS is a broadcast only service.
The GPS was conceived, designed and deployed as a military force enhancement. Consequently much of the capability of the GPS (i.e. the Precise Positioning Service or PPS) is not available to Civil users. Furthermore, even the GPS SPS service which is available to the civil community was not designed with adequate integrity, reliability or availability necessary to support safety of life civil applications. Furthermore, the SPS includes a relatively low power signal on only a single frequency and is consequently vulnerable to intentional or unintentional interference. These problems with the integrity and robustness of the civil GPS services are well known in the art.
As the SPS signals travel from the GPS satellites to the GPS receivers the SPS signals travel through the ionosphere which encircles the earth. The ionosphere acts as a dispersive medium and refracts the SPS signals as they travel through the ionosphere. As a result, the SPS signals do not appear to travel at the speed of light, which is assumed in the calculation of the position of the GPS receiver. The ionospheric induced delay in the reception of the SPS signals limits the accuracy of the determination of the position of the GPS receiver and is the largest location dependent error source in the calculation of the position of the GPS receiver. Therefore, the use of the GPS SPS signals to compute a position of the GPS receiver has limited accuracy and cannot be used for applications requiring a high degree of precision in the determination of the position of the GPS receiver.
To overcome the aforementioned shortcomings of the GPS, a number of space based augmentation systems (SBAS) are under development. For example, there are currently three SBAS systems under development worldwide: the Wide Area Augmentation System (WAAS) under development by the Federal Aviation Administration; the European Geostationary Navigation Overlay Service (EGNOS) under development by the European Space Agency in conjunction with EURO CONTROL and the European Union; and the MTSAT Satellite Augmentation System (MSAS) under development by the Japanese Civil Aviation Bureau. These SBASs provide for a way to measure and correct for the ionospheric delay caused by the SPS signals traveling through the ionosphere on its way toward earth and provide for basic integrity monitoring of the GPS SPS service sufficient to meet the requirements for civil aviation applications. However, all these SBAS will operate on the same GPS L1 frequency and will ultimately depend on the availability of basic GPS SPS. Hence SBAS does little or nothing to address the robustness concerns of GPS.
The electron density of the ionosphere varies as a function of geographic location. In a vectorized, wide area differential solution such as that employed by an SBAS, a large number of sampling locations are needed to compute an accurate model of the variation of the time delay induced in a signal traveling through various locations in the ionosphere. Therefore, in order to get adequate sampling of the state of the ionosphere, the SBASs employ a number of reference stations over a wide region that are fixed to the earth. These reference stations are connected via a ground based telecommunications network to a central processing facility. Each reference station observes the transmitted SPS signals from the GPS satellites visible at the reference station, performs some signal integrity monitoring, and passes the data on to the central processing facility via the ground based telecommunications network. These stations also track a component of the PPS using a codeless tracking technique in order to make dual frequency measurements of the ionosphere. The central processing facility uses the data from the reference stations to compute xe2x80x9cwide-areaxe2x80x9d differential corrections where separate corrections are given for various satellite pseudo range error components. The SBASs then provide estimates of the vertical ionospheric delay at predefined grid points over the region covered by the SBAS to users of the SBAS. The estimates are broadcast from the SBAS to the user via a satellite link which is designed to be very similar to a GPS signal. The GPS receiver can then compute an estimate of the ionospheric delay for each pseudo range based on the user""s location and the geometry of the satellites and compute its position more accurately by accounting for the ionospheric delay in the SPS signals and by applying the other differential correction components included in the SBAS signal.
The SBAS architecture is attractive in that it supports operations over a wide area and may even be capable of providing a level of service sufficient to support category 1 precision approach aircraft operations. However, the complexity and cost of such a system makes it impractical for most States or regions to consider employing such a system. Particularly, the cost of the ground based telecommunications network can be very significant. Also, in order to get good sampling of the ionosphere and a more accurate grid of the errors introduced by the ionosphere, a large number of reference stations are required, which in turn increases the cost of connecting all the reference stations with the ground based telecommunication networks.
Therefore, it is desirable to develop a system and method for accurately measuring and correcting the time delay induced in signals traveling through the ionosphere. Additionally, it is desirable to perform the ionospheric delay sampling without the need for an extensive network of ground based monitoring stations. It is also desirable to provide monitoring stations without the need for the monitoring stations to be connected to the central processing facility by expensive ground based telecommunication networks.
The present invention is directed to a method and apparatus for providing a global communication, navigation and surveillance (GCNS) system that overcomes the shortcomings of the GPS without the need for an extensive SBAS. Additionally, the present invention provides for an entirely new class of capabilities heretofore unavailable with either the GPS or SBAS.
In one preferred embodiment, the GCNS system of the present invention makes use of a plurality of time synchronized satellites. Each satellite broadcasts multiple navigation signals, can engage in two-way communications, and can receive and relay surveillance signals. Each satellite of the plurality of satellites is part of a network that allows each satellite of the plurality of satellites to communicate with any other satellite of the plurality of satellites. There is a terrestrial segment that has a processing apparatus that is capable of two way communication with any satellite of the plurality of satellites through the network. There is also at least one mobile user device that is capable of two-way communication with the plurality of satellites. The at least one user device can directly communicate with each satellite of the plurality of satellites that are within a line of sight of the user device and with the remaining satellites through the network. The at least one user device is also capable of receiving the navigation signals broadcast by the plurality of satellites and computing a position of the at least one user device based on the received navigation signals. The at least one user device can broadcast a surveillance signal to the plurality of satellites so that the position of the at least one user device can be computed by the processing apparatus.
Preferably, the surveillance signal broadcast by the at least one user device is a dual frequency surveillance signal which the processing apparatus uses to compute correction factors for ionospheric induced time delays in signals traveling through the ionosphere between the at least one user device and each satellite of the plurality of satellites that received the surveillance signal. The processing apparatus using the correction factors and the surveillance signals can compute a more accurate position of the user device. Preferably, the correction factors are transmitted to the at least one user device so that the at least one user device can use the correction factors along with navigation signals to compute a more accurate position of the at least one user device. The GCNS system thereby provides surveillance capabilities for the plurality of satellites that correct for ionospheric delay and also provides the at least one user device with correction factors so that a more accurate position of the at least one user device can be computed by the at least one user device.
Preferably, each satellite of the plurality of satellites has communication switching capabilities so that each satellite of the plurality of satellites can route communication signals to a desired recipient. The network can be formed by having each satellite of the plurality of satellites directly communicating with at least two other satellites of the plurality of satellites, with at least two ground stations, or with at least one other satellite of the plurality of satellites and at least one ground station so that redundant communication paths exist and each satellite of the plurality of satellites is capable of communicating with any other satellite of the plurality of satellites either directly or through the network.
Preferably, the at least one user device is one of a plurality of user devices with each user device of the plurality of user devices providing dual frequency surveillance signals to the plurality of satellites. The processing apparatus is capable of using the dual frequency surveillance signals to compute a model which describes variation of an ionospheric induced time delay in signals traveling through the ionosphere as a function of geographic location. Correction factors for the ionospheric induced time delay in signals passing through the ionosphere are computed for each line of sight between the plurality of user devices and the plurality of satellites that receive the surveillance signals. The model along with the correction factors are broadcast by the plurality of satellites so that a device capable of receiving and processing these broadcasts can use the navigation signals along with the model and correction factors to compute a more accurate position of the device. The GCNS system thereby provides a map of the ionosphere along with correction factors to allow for increased accuracy in the determination of the position of one of the user devices without the need for extensive use of ground based monitoring stations. Because the ionospheric delay scales linearly with frequency, the ionospheric delay model broadcast by the system can be used to correct for ionospheric delay on any frequency used by the system. Consequently, improved accuracy can be achieved for single frequency navigation or surveillance users.
The GCNS system also provides the ability to verify the accuracy of the position determined by a user device. The user device can broadcast its computed position (based on received navigation signals) along with the surveillance signals. The processing apparatus can use the surveillance signal broadcast by a user device to compute a position of the user device. The location of the user device based on the surveillance signals can be compared to the reported position of the user device to determine the difference between the two computed positions. This comparison provides a degree of integrity checking to the system. If the positions differ by more than a predetermined amount an error is probably occurring somewhere in the system and the processing apparatus can perform a system integrity check of the plurality of satellites to verify that each satellite is broadcasting correct navigation signals. Additionally, the GCNS system can notify the user device of the difference between the two computed positions and whether the system integrity has been verified so that the user can have a correct position of the user device.
Monitoring stations can be provided that are fixed on the earth at known positions and are capable of receiving the navigation signal and of two-way communication with the plurality of satellites. The monitoring stations can operate similarly to the user devices. The monitoring stations can monitor the navigation signals and compute the indicated position of the monitoring station based on the navigation signals so that the integrity of the system can be checked. The monitoring stations can also broadcast dual frequency surveillance signals to the satellites so that the delay in signals travelling through the ionosphere between the monitoring stations and the satellites that receive the surveillance signals can be measured and corrected for. In this way, adequate sampling of the ionosphere can be achieved even in geographic regions where user densities are too low to otherwise provide a large enough number of ionospheric delay observations.
Additional capabilities are also realized with the GCNS system. When a user device is not able to receive the navigation signals being broadcast by the plurality of satellites the user device can communicate to the GCNS system that it is not receiving the navigation signals. The processing apparatus can then perform a system integrity check to ensure that the plurality of satellites are properly broadcasting the navigation signals. If the plurality of satellites are found to be operating correctly, then either the user device is malfunctioning or there is an interference source that is interfering with the reception of the navigation signals by the user device. When there are a plurality of user devices within a region that report not receiving navigation signals and the satellites are found to be operating correctly, the processing apparatus can use the positions of the plurality of user devices (computed based upon surveillance signals sent by the user devices) to determine the probable location of an interference source that is preventing the reception of the navigation signals. The GCNS system can then report the problem and the probable location of the interference source to a desired recipient such as a state""s frequency management authority. In this manner, the operation of the GCNS system can be continually monitored and probable locations of interference sources can be determined.
The GCNS system provides a robust navigation capability because the tightly coupled communications, navigation and surveillance capabilities allow the surveillance and navigation capabilities to act as a backup for each other. For example, if one of the plurality of user devices is unable to receive the navigation signals broadcast by the plurality of satellites, the system can use the surveillance function to obtain a position fix for the user and provide the position fix to the user over the communications link. A backup navigation mode is realized in this manner. Similarly, if the system is unable to perform the surveillance function for a particular user device, then the user device may broadcast its position as determined from the navigation signals broadcast from the satellites. A backup surveillance mode is realized in this manner.
Another benefit realized by the GCNS system is that the processing apparatus is capable of computing the ephemeris of each satellite of the plurality of satellites based upon signals broadcast by each satellite of the plurality of satellites. Preferably, the processing apparatus uses the computed ephemeris of each satellite of the plurality of satellites to compare it with the navigation signals being broadcast by each satellite of the plurality of satellites to ensure that the plurality of satellites are broadcasting correct navigation signals. Because the satellites communicate with each other, the processing apparatus can correct the navigation signals being broadcast by each satellite of the plurality of satellites found to be in error. The GCNS system can thereby autonomously monitor and correct itself when incorrect navigation signals are being broadcast by one of the satellites.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.